1-hexene and 1-octene are important commercial raw materials, which are widely used as monomers or comonomers in polymerization processes to produce linear low-density polyethylene, and are obtained by purifying products of ethylene oligomerization. However, ethylene oligomerization reactions conducted to date are inefficient in that 1-hexene and 1-octene are produced together with significant amounts of butene, higher oligomers and polyethylene. In such prior ethylene oligomerization technology, the yield of a desired product is limited, because a range of α-olefins is generally produced following a Schulze-Flory or Poisson product distribution.
Recently, studies focused on either producing 1-hexene by selectively trimerizing ethylene through transition metal catalysis or producing 1-octene by selectively tetramerizing ethylene have been conducted, and most known transition metal catalysts for use in ethylene trimerization or tetramerization are chromium-based catalysts.
International Patent Publication WO 02/04119 discloses a chromium-based catalyst for ethylene trimerization, which comprises a ligand of the formula (R1)(R2)X—Y—X(R3)(R4), wherein X is phosphorus, arsenic or antimony, Y is a linking group such as —N(R5)—, and at least one of R1, R2, R3 and R4 has a polar substituent or an electron donating substituent.
Another publication discloses the use of (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2, a ligand which shows no catalytic activity for 1-hexene under catalytic conditions and has no polar substituent in at least one of R1, R2, R3 and R4 (Antea Carter et al., Chem. Commun., 2002, p. 858-859).
Also, Korean Patent Laid-Open Publication No. 2006-0002741 discloses that excellent activity and selectivity of ethylene trimerization can be achieved in practice through the use of a PNP ligand containing a non-polar substituent at the ortho position of a phenyl ring attached to phosphorus, for example, (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2.
Meanwhile, International Patent Publication WO 04/056479 discloses enhancing selectivity in a process of producing 1-octene by tetramerizing ethylene using a chromium-based catalyst containing a PNP ligand having no substituent on a phenyl ring attached to phosphorus. In the patent publication, examples of a heteroatom ligand which is used in the catalyst for ethylene tetramerization include (phenyl)2PN(isopropyl)P(phenyl)2, etc.
This prior publication discloses that the chromium-based catalyst, containing a heteroatomic ligand having both nitrogen and phosphorus heteroatoms, without any polar substituents on the hydrocarbyl or heterohydrocarbyl groups on the phosphorus atom, can be used to selectively tetramerize ethylene to produce 1-octene, often in excess of 70 mass % selectivity.
However, the prior publications do not suggest a concrete example of a heteroatom-containing ligand structure, which can tetramerize ethylene at high selectivity to produce 1-octene or trimerize ethylene at high selectivity to produce 1-hexene. Also, these publications suggest only a PNP-type framework structure, such as (R1)(R2)P—(R5)N—P(R3)(R4), which is a ligand having a selectivity to 1-octene of about 70 mass %. Moreover, the possible substituents for heteroatomic ligands are also limited.
In addition, the prior PNP-type backbone ligands containing heteroatoms have problems in that their activity in reactions for producing 1-octene or 1-hexene changes over time, and the reaction rate is greatly reduced.